Federation bThe well-established killing of bacteria by copper surfaces, also called contact killing, is currently believed to be a combined effect of bacterial contact with the copper surface and the dissolution of copper, resulting in lethal bacterial damage. Iron can similarly be released in ionic form from iron surfaces and would thus be expected to also exhibit contact killing, although essentially no contact killing is observed by iron surfaces. However, we show here that the exposure of bacteria to iron surfaces in the presence of copper ions results in efficient contact killing. The process involves reduction of Cu 2؉ to Cu ؉ by iron; Cu ؉ has been shown to be considerably more toxic to cells than Cu 2؉ . The specific Cu ؉ chelator, bicinchoninic acid, suppresses contact killing by chelating the Cu ؉ ions. These findings underline the importance of Cu ؉ ions in the contact killing process and infer that ironbased alloys containing copper could provide novel antimicrobial materials.T he killing of bacteria by metallic copper surfaces, so-called "contact killing," is now well established and has explicitly been shown for many species (1). Bacteria are killed within minutes on surfaces of copper or copper alloys containing at least 60% copper. In contrast, cells can survive for days on surfaces of stainless steel, glass, or plastics. Copper and copper alloys have attracted attention as a means of creating self-sanitizing surfaces in the light of increasing nosocomial infections in Western hospitals. In a number of hospital trials, rooms have been fitted with copper alloy table tops, bedrails, door handles, light switches, bathroom fixtures, etc., in an effort to curb nosocomial infections (2-5; K. Laitinen et al., unpublished data). These copper surfaces resulted in a 2-to 3-log reduction of the microbial burden on a continuous basis. However, further data are needed to convincingly demonstrate that these measures also lead to a lasting reduction of nosocomial infections. Nevertheless, it appears clear that copper-containing materials can contribute to hospital hygiene and lower the bacterial burden also in other facilities where clean or aseptic working procedures are required (6).The mechanism of contact killing of bacteria by coppercontaining materials is of interest not only in connection to its use in hospitals but also from a purely scientific point of view. Laboratory studies have shown that bacteria on copper surfaces suffer rapid membrane damage and DNA degradation, in addition to other less well-defined cellular damage (7-12). The importance and the order of the different processes leading to cell death may depend on the type of microorganism (9). One key element required for contact killing is the release of copper ions from the metal surface. Bacterial copper resistance systems appear unable to cope with the released copper (13-15). The second important requirement for contact killing is bacterial contact with the metal surface (16). Recently, we showed that bacteria are also killed effectively ...